The problem in which we can't tell which of two variables causes the other (e.g., does A cause B or does B cause A) is called the third-variable problem.
A major disadvantage of correlational research is that the correlation coefficient tells you the direction of a correlation (i.e., whether X and Y are negatively or positively correlated) but not how strongly the two variables are related.
The independent variable is the variable administered to the experimental group and the dependent variable is the variable administered to the control group.